1. Field of Invention.
This invention relates to a nuclear magnetic resonance imaging apparatus, and more particularly to improvements therein involving a novel pulse sequence and novel processing of data to improve the signal to noise ratio.
2. Description of Prior Art.
Nuclear magnetic resonance (called "NMR") imaging apparatus are known in the art and can examine a plane section of an object by using a planar method, namely, a Fourier transformation method (called "FT" method). Such Fourier transformation methods may be divided into two types, (1) FID method wherein an FID signal is measured and is then Fourier transformed, and (2) Spin Echo method wherein a spin echo signal is measured and is then Fourier transformed.
In the FID method, an FID signal is obtained by a pulse sequence, such as illustrated in FIG. 6. Specifically, having been given a Z gradient magnetic field G.sub.z.sup.31 (FIG. 6, line (d)), a plane of the object is excited by impressing thereon a 90.degree. RF pulse, such as shown in FIG. 6, line (a). Then, Z gradient magnetic field G.sub.z.sup.+ and Y gradient magnetic field G.sub.y (FIG. 6, lines (c) and (d)) are impressed, and thereafter the X gradient magnetic field G.sub.x is impressed while terminating the Z gradient magnetic field G.sub.z.sup.+ and the Y gradient magnetic field G.sub.y. Then, the FID signals are measured (FIG. 6, line (e)).
In the Spin Echo method, echo signals are obtained by a pulse sequence as shown in FIG. 7. Specifically, having been given a Z gradient magnetic field G.sub.z.sup.- (see FIG. 7, line (d)), a plane of the object is excited by impressing on the object a 90.degree. RF pulse (FIG. 7, line (a)), then a Z gradient magnetic field G.sub.z.sup.+ (FIG. 7, line (d)) and a Y gradient magnetic field G.sub.y (FIG. 7, line (c)) are impresed, and thereafter an X gradient magnetic field G.sub.x (FIG. 7, line (b)) is impressed while terminating the Z gradient magnetic field and the Y gradient magnetic field. During the impression of the 180.degree. RF pulse (FIG. 7, line (a)), X gradient magnetic field G.sub.x is terminated (FIG. 7, line (b)). After that, the X gradient magnetic field G.sub.x is impressed on the object again. Although spins are still in the relaxation state, the spin echo signal becomes maximum after t time period from the 180.degree. RF pulse and then attenuated, as shown in FIG. 7, line (e).
To accomplish Fourier transformation, it is necessary that the data at the positive and negative times be zero at the center time point of a signal.
However, in the FID method, zero time point is not apparent (t.sub.FID of FIG. 6 is not the zero time point), and phase rotation (which is different according to the area) occurs due to non-uniform distribution of magnetic field by the time t=t.sub.FID from the time of excitation of a 90.degree. RF pulse (i.e. from the center of the 90.degree. RF pulse on a time scale) to the FID signals, and it is difficult to assume data (that is to produce the data) of the negative time. Consequently, one problem with this method is that the reconsitituted image is subjected to deformation and is unclear.
On the other hand, in the Spin Echo method, data assumption or preparation is not necessary because the center of the echo signal (the point of time of 2t time from the application of the 90.degree. RF pulse). However, even after 2t time, the phase displacement cannot be focused based on the original transverse relaxation, and the absolute amount of magnetization is reduced in correspondence to T.sub.2 attenuation, and the signals become smaller thus causing a disadvantageous S/N ratio.